(1) Field of the Invention
The present invention relates to valve caps and in particular, though not exclusively, to a valve cap for use on a hole in a mud-pump fluid-end module.
(2) Description of Related Art including information disclosed under 37 CFR 1.97 and 1.98
In the oil industry mud pumps are used to pump viscous fluids, such as drilling muds, cement, or other well fluids. Although mud pumps may be either centrifugal or reciprocating type pumps, typically mud pumps are reciprocating pumps using one or more pistons and hydraulic cylinders with liners to generate the high pressures required to pump these viscous fluids in and out of the well.
Mud pumps include a fluid end and a power end. At the fluid end, low pressure fluid is drawn in and built-up by compression via a pump piston and check valves, until the pressure overcomes well bore pressure so as to pump the mud into the well. The power end contains the gears that reciprocate the pump piston. It will be appreciated that parts within the pump exposed to the fluid and its associated pressure are liable to wear easily. In particular sufficient seals need to be provided at unused inlets/outlets and at the valves.
These seal covers are typically referred to as valve caps or valve covers. They must provide a seal while closing off the aperture of an end piece at the fluid end of the pump.
FIG. 1 shows a prior art valve cap A for use with a pump as supplied by Southwest Oilfield Products, Inc, Houston, Tex., USA. A valve plug B is located against a step in the aperture C of an end piece D. A seal E is provided between the parts. The seal is maintained by pressure from a cap body F located against it. The body F is screwed in place through a locking member G attached to the end piece D at an end face H. Once located the locking member G is forced against the end face H by using a stud rods J and retention nuts K, L as is known in the art. This movement is transferred to the body F via the screw threads and effectively locks the body F against the plug B. When the cap A needs to be removed the nuts K, L are released and a steel bar is inserted through a guide hole M in the body F and turned to remove the body F and release the plug B
A disadvantage of this valve cap is in the use of threaded connections. It is difficult to determine if the threads are correctly tightened. During mud pump operation, the reciprocating nature and peak pump pressures acts on any insufficiently tightened connections, resulting in a tendency for the valve cap to gradually loosen. Alternatively, the threaded connections have been over tightened, making it even more difficult to unthread. Additionally, in using a steel bar it is often necessary to hammer the bar to release the cap. Such activity is obviously dangerous. In some regions of the world local laws prohibit the use of sledge hammers for personnel safety reasons.
To overcome these problems a spring based retaining valve cap has been developed. This valve cap is illustrated in FIG. 2 and is supplied by P-Quip Ltd., Linwood, Scotland. Like parts to those of FIG. 1 have been given the same reference. In this cap, the body F is forcibly pushed against the plug B by a number of piston and spring arrangements located in the locking member G. The member G is initially bolted to the end piece D at the face H by bolts M. Each arrangement comprises a cylinder N adapted to house a slidable piston P and clamping springs Q. The piston P has a threaded rod R extending outwith the cylinder N and through the body F. A retaining nut K is located on the threaded rod R. In use, the cap is assembled as shown in FIG. 2 with the nuts K on the threaded rods R. Hydraulic fluid is then inserted between the piston P and the cylinder base, such that the piston P is extended to a greater extent outwith the cylinder N and the nut K is tightened further against the body F. The hydraulic pressure is then released and the springs Q apply their force to the plug B through the rods R, the nuts K and the body F.
A disadvantage of this cap is in the large dimensions of the cap and the respective face on the end piece required. This is because the space must be available both for bolts to connect the locking member to the end piece, and for the cylinders in which the pistons are housed. As a result these caps are generally limited to a maximum of four cylinders which has the disadvantage of causing an uneven pressure to be applied to the body.